1. Field of the Invention
The present invention relates generally to the field of fuel-air mixture controls in flames and, more specifically, to a method for controlling burner efficiency by utilizing a control parameter based on a known relationship between the fuel number and emission radiation detected within the fuel flame.
2. Description of the Prior Art
Prior art has offered various optical methods to monitor and regulate the fuel-air mixture in burner flames. However, these methods either implement a complex control factor to allow for variable flame conditions or rely on simplifying assumptions which undermine the validity of the control parameter.
One such method and apparatus is described in U.S. Pat. No. 4,435,149. That system uses a type of radiation detection to derive a control parameter for fuel-air mixture in furnace flames which also compensates for variations in the flame path due to fluctuations in burning rate in an attempt to insure consistent monitoring of burner efficiency in a furnace having a variable firing or burning rate. Radiation detectors are used to sense carbon dioxide in the products of combustion. More particularly, wavelength bands are detected, namely, the strong and weak bands for carbon dioxide and a third band representing one of nonabsorption for any chemical species in the combustion products. The control ratio is derived from the strong CO.sub.2 band and the nonabsorbing band emissions, while the weak CO.sub.2 band emission is used to compensate for varying flame length due to varying load conditions.
However, this method has certain drawbacks. It is restricted to the use of CO.sub.2 emission bands and, therefore, actually presents but another, more sophisticated, form of CO.sub.2 emission radiation related controller which is common in the prior art. Furthermore, this method also assumes that particulate concentration within the flame is solely dependent on excess oxygen. It has been found, however, that this assumption fundamentally over-simplifies the parameter of particulate concentration under many circumstances, thereby imparting a shortcoming to the primary basis for calculating the control parameter. This, in turn, may lead to the introduction of serious error in results which, in this case, renders consistent monitoring of burner efficiency quite difficult.
On the other hand, the method of the present invention does not depend on such assumptions and accurately monitors burner efficiency regardless of fluctuating flame conditions and natural interferences thereby overcoming problems plaguing the prior art. The present method contemplates computation of a control parameter using any number of chemical species that emit radiation within the flame. The control parameter is based on the relationship between fuel number and the concentration of various chemical species within the flame. The relative intensity of radiation emitted by any given excited chemical species or emitter is indicative of concentration. The relative emission intensities of any two chemical species may be used to calculate the control parameter. This control parameter enables a burner to attain and maintain an optimum predetermined fuel number.
Unlike some prior methods, this method is not restricted to detection and control by monitoring a definite single species such as CO.sub.2, provided the flame gases are optically thin in the spectral region being used. Any chemical species, occurring within the flame, as a combustion product of the burned fuel, can be used in the monitoring of flame efficiency. Furthermore, this control parameter does not require correction for fluctuating flame conditions. The persistent presence of quantum sufficient radiation eminating from the chemical combustion products allows accurate flame monitoring even with turbulent conditions, varying particle luminescense, varying flame shape, varying site, or varying burning rates. It is not insignificant that the underlying concept, that radiation intensity indicative of chemical species concentration correspondingly varies with fuel number, remains valid despite the presence of interferences. Finally, this method does not require any simplifying assumptions. The correlation between radiation intensity and fuel number is well documented. Thus, the present method provides positive monitoring and regulation of fuel number by a control parameter based on the relationship between fuel number and the intensity of radiation emitted from selected specific chemical species within the burner flame.